Works great Rated 5 out of 5 stars
I've been using CountdownClock in Fx2 for years; works fine in Fx3.5, too. I do, however, prefer the old "button" interface to the new black "pointers". (Couldn't you just have put pointers on the button faces?) Also, I'd prefer a single left-click to open the setup/options window instead of a double click. (I mean, why not?) Only need one "timer" per Firefox; for more timers... well, that's what calendar apps are for.